by David Ryan, Senior Business Development and Strategic Marketing Manager – MACOM

The evolution to 5G mobile networks continues to accelerate, promising breakthrough gains in wireless throughput and capacity. In the near term, we’ll see sub-6 GHz wireless infrastructure begin to be deployed to bridge the bandwidth gap between existing 4G LTE networks and future millimeter wave (mmW) 5G implementations that leverage frequencies significantly higher than 6 GHz.

Sub-6 GHz infrastructure will continue to take advantage of the significant amount of available spectrum from 2.5 to 2.7 GHz, adding frequencies between 3.3 and 3.8 GHz, and, in some geographies at least, from 4.4 to 5 GHz. With major trial deployments planned for 2017 and 2018 by China Mobile, pre-5G sub-6 GHz infrastructure promises to increase spectral efficiency for legacy cellular bands, and expand capacity and coverage at data rates that are up to 10X faster than existing 4G LTE in comparable frequency bandwidth. 5G wireless infrastructure at sub-6 GHz will be deployed broadly, leveraging beam forming schemes that dramatically expand network reach and in-building penetration.

The 3GPP Consortium’s first set of 5G standards (Release 15) is not expected to be ratified until June 2018, and mainstream commercialization of 5G at mmW frequencies will not begin for a few years, but demonstration systems and pre-standards are in development today and some significant milestones have been achieved. Verizon and AT&T have announced early tests/trials for deploying 5G mmW technology for fixed wireless applications to compete with traditional cable operators and to deliver the bandwidth required to support multiple simultaneous 4K video per household. 5G will also likely be used to provide massive capacity in densely populated environments like stadiums and metro shopping centers, for example. Future use cases will become more apparent as the technology evolves.

5G represents more than just a faster network at higher frequencies, however. Among its key features, 5G will enable operators to monetize their networks in new ways and evolve their business models via new capabilities such as federated network slicing. With this ability to segment a physical network into several virtual mobile networks, operators can provide a wide range of quality of service (QoS) and security/encryption options to corporate customers, utilizing the same hardware infrastructure used by consumer subscribers. Looking ahead, federated network slicing could also enable greater platform sharing among operators, giving them the ability to cooperatively allocate network slices to each other across continents to enable seamless 5G roaming for subscribers.

Massive MIMO, Myriad Challenges

Sub-6 GHz and mmW 5G systems will rely on phased array technology to optimize signal link and data rate, leveraging large numbers of antennae elements configured in massive MIMO (multiple input, multiple output) architectures. Whereas conventional basestations may house between two and eight transmitters and receivers, massive MIMO systems can be populated with 64 transmit and receive (T/R) elements, and could scale up to 128 or 256 elements. These arrayed antennae configurations increase the number of available T/R paths to maximize data rates, and enable the advanced beamforming capabilities that are central to the 5G value proposition — but the complexity and density of these systems pose several design and assembly challenges.

Massive MIMO systems require compact front-end solutions, given how the element-to element spacing decreases within tightly clustered antenna configurations, particularly at higher frequencies. This in turn creates thermal challenges associated with generating significant RF power (in some cases up to 5W per element) and dissipating the heat in a small area.

Another major challenge is the assembly of the final unit. A 64-antennae array will host 64 PAs, 64 switches, 64 LNAs, etc. This huge number of RF components and RF interfaces creates significant risks of poor final build yield. While some basestation OEMs are equipped to assemble thousands of components and handle the PCB packaging in-house, other OEMs opt for reduced complexity and yield risk, procuring fully assembled modules to serve as functional blocks within their radios. By leveraging higher level assemblies, component failures are isolated to one of 64 subsystems, making it far easier to rework the board compared to an assembly comprised of thousands of individual components, compromised by a single failure.

Gen4 GaN Advantages

At the semiconductor level, 4th generation GaN on Silicon (Gen4 GaN) has emerged as the clear successor to LDMOS for next generation basestations targeted for 5G deployments, particularly at frequencies of 3.5 GHz and higher where LDMOS is constrained by its inherent technology limitations. Gen4 GaN’s technology leadership over LDMOS has already been established in 4G LTE infrastructure, providing clear advantages in power density, space savings and energy efficiency, on a pathway toward better-than-LDMOS cost structures.

Gen4 GaN delivers considerably higher raw power density than incumbent LDMOS technology, providing 4X to 6X more power per unit area; this translates into GaN die sizes that are between 1/4 and 1/6 the size of LDMOS dies. The higher power density provided by Gen4 GaN naturally enables smaller device packages, making it ideally suited for use within massive MIMO antenna systems.

Gen4 GaN also provides more than 10 percentage points greater efficiency than LDMOS. When properly exploited, this efficiency delta will have a big impact at the system level in commercial 5G applications, particularly in higher level assemblies where multiple packaging layers demand a thermally adept solution like Gen4 GaN that can operate at higher junction temperatures.

Lastly, Gen4 GaN allows device designers to achieve broad bandwidths, which is important as operators move to higher frequencies where the bands are wider, giving them the flexibility to implement more carrier aggregation bands. Since GaN-based PAs cover much wider bandwidth than LDMOS-based devices, they reduce the number of parts needed to cover the major cellular bands within 5G basestations.

MPAR Assembly Efficiency

When it comes to the architecture and assembly of massive MIMO 5G systems, we see many parallels with the new generation of Multifunction Phased Array Radar (MPAR) systems targeted for use for military and civil air traffic control applications. Sub-6 GHz massive MIMO systems are particularly well positioned to leverage MPAR design and assembly strategies, given that both technologies cover frequency bands from 2.6 to 3.5 GHz, and share a 64-antennae architecture.

First generation MPAR systems leverage an array of Scalable Planar Array (SPAR™) Tiles in a flat panel configuration comprised of hundreds to thousands of T/R elements. SPAR Tile technology, developed in a collaboration between MACOM and MIT Lincoln Laboratory, embodies a new cost-conscious approach to phased array radar system development, leveraging higher level RF assemblies and volume scale commercial packaging and manufacturing techniques.

Figure 1: MPAR board as seen from above

SPAR Tiles eschew conventional slat array architectures in favor of a planar, tile array architecture whereby the antennae elements and RF beamformers are integrated into a single multilayer RF board. With this approach, T/R modules are SMT mounted to the PCB using industry standard manufacturing processes, streamlining system assembly, and minimizing yield risks. This style of phased array implementation shortens time to market and reduces costs dramatically, propelling MPAR technology toward mainstream adoption for commercial applications like sub-6 GHz wireless.

Conclusion

5G systems at sub-6 GHz and mmW frequencies pose several unique design challenges, from the semiconductor level to device packaging and final system assembly. Continued innovations in GaN and phased array-based technologies like MPAR will help to unlock the full promise of 5G, allowing basestation OEMS to achieve an optimal balance of power output and energy efficiency in compact form factors, leveraging modular subsystems that simplify design and manufacturing processes.